As a method for fabricating an SOI wafer, a wafer bonding method and an SIMOX method are generally known. The wafer bonding method is a method of bonding, e.g., two silicon wafers through an oxide film without using an adhesive, increasing bonding strength by a heat treatment (1000 to 1200° C.), and then reducing a film thickness of one wafer based on, e.g., grinding/polishing or etching, and this technique is advantageous in that crystallinity of the SOI layer or reliability of a buried oxide film (a BOX layer) is equivalent to that of a regular silicon wafer but disadvantageous in that film thickness uniformity of the SOI layer has a limit (approximately ±0.3 μm at most) and its cost is high since two silicon wafers are used for manufacture of one SOI wafer.
Further, Patent Document 1 suggests an ion implantation and delamination method (which is also called a smart cut (a registered trademark) method) as one of the bonding methods. This method is a method of forming an oxide film on at least one of two silicon wafers, implanting at least one type selected from hydrogen ions and rare gas ions into one main surface of one wafer to form an ion implanted layer in the wafer, then closely attaching the ion implanted surface and one main surface of the other silicon wafer through the oxide film, and thereafter performing a heat treatment at a temperature of 300° C. to 600° C. or a higher temperature to effect delamination at the ion implanted layer, and this method has superiority in easily fabricating a thin SOI wafer having SOI layer film thickness uniformity of ±10 nm or below and superiority in reusing the delaminated bond wafer more than once to achieve a reduction in cost.
On the other hand, the SIMOX method is a method of implanting high-concentration oxygen ions into a silicon wafer to form an oxygen ion implanted layer, and then performing annealing processing at a high temperature of approximately 1300° C. to form a buried oxide film (a BOX layer) in the silicon wafer for using a layer on the surface side as an SOI layer. Although manufacture based on the SIMOX method is easy, the BOX layer formed from the oxygen ion implanted layer is restricted to the outermost surface layer and cannot be formed at a deep position of the wafer, and hence increasing a thickness of a surface layer device region is difficult. Furthermore, the formed BOX layer does not have a dense configuration, and this method has a drawback that a perfect dielectric strength voltage that is the greatest merit when using an SOI wafer as a device fabrication wafer is hardly obtained.
Meanwhile, although a thick SOI wafer having an SOI layer whose film thickness falls within the range of several μm to several-ten μm is a very useful wafer for a bipolar device or a power device, it is known that fabricating a high-quality SOI wafer at a low cost is difficult even though the bonding method using the grinding/polishing and the smart cut method. That is because, the bonding method using the grinding/polishing needs bonding a wafer having an oxide film and a bare wafer, carrying out a bonding heat treatment at 1100° C. or above, and performing grinding and polishing processing to obtain a desired SOI layer thickness, and hence the process becomes complicated and improving film thickness uniformity of the SOI layer is very difficult. On the other hand, in case of the smart cut method, a thickness of an SOI layer is determined based on a depth where ion implantation is possible (i.e., an acceleration voltage of an ion implantation device), a maximum acceleration voltage is approximately 200 keV in general implantation devices, and an SOI layer having a thickness of approximately 2 μm at most can be obtained.